Feline Infectious Peritonitis (FIP)

What is Feline Infectious Peritonitis (FIP)?

FIP is a fatal disease in cats and is caused by a mutation of the feline corona virus. Infection with corona virus is actually very common in cats (fabcats.org estimates some 25 - 40 per cent of household pet cats are infected, rising to 80 - 100 per cent of cats kept in multi-cat households, rescue and breeding colonies) but usually when it occurs, it is benign; that is to say, it doesn't cause any problems for the cat other than perhaps a mild bout of diarrhoea. However, in approx 3% of cats, this benign virus mutates, causing FIP, a deadly auto-immune disease.

Hence, although FIP is caused by the corona virus, infection with the corona virus doesn't necessarily mean that FIP will develop - this mutation only occurs in a very small number of cases.

It isn't yet fully understood why this mutation occurs, but several factors are likely to play a role. A lot of cases of FIP occur in younger cats and many cases probably arise straight after their first infection. The corona virus is prone to mutation and mutations are most likely to occur right after infection (when the highest amount of viral replication occurs) - increasing the risk that the relatively benign intestinal corona virus is mutated into the fatal FIP virus. Other factors present in younger cats such as a less-developed immune system, combined with stressors such as neutering, vaccinations and recent re-homing could all contribute to the likelihood of the virus mutating. The peak age for development of FIP is between 6 months and 2 years old.

That said, FIP can occur in any age group and predisposing factors or risk factors aren't always evident. There is also a possibility that genetics plays a part because certain breeds of cats are statistically more likely to develop FIP than others.

Unfortunately, the disease is fatal - there is no cure. Supportive treatment may give temporary relief to symptoms, depending on the physical condition of the cat but eventually the disease will advance to a stage where treatment no longer works.

How is FIP transmitted?

Corona viruses affect many different species. In dogs and pigs, they cause intestinal and respiratory disease; in humans, they are a cause of the common cold. Feline corona viruses are related to these other corona viruses. In cats, the intestinal (enteric) form of the virus is the most common. This intestinal form of the corona virus is easily transmitted between cats - via by the faecal-oral route. This means that the virus is shed via the faeces into the environment and that cats then become infected when they ingest the virus while grooming or eating. The majority of infected cats tend to shed the virus via their faeces for a certain period of time (which varies cat to cat) and then stop. Other cats shed the virus continuously.

Corona viruses are prone to undergoing genetic change (i.e. mutate) and it is thought that sometimes this mutation results in FIP. The comparatively harmless intestinal corona virus mutates into the fatal FIP virus.

Cats infected with Feline Leukaemia Virus (FeLV) and / or Feline Immunodeficiency Virus (FIV) are more likely to develop FIP.

What are the symptoms?

Symptoms of feline corona virus infection are mild - some diarrhoea, and sometimes some vomiting, lasting a few days. With FIP, there are no symptoms that are unique to the disease. However, generally, FIP will present in one of two forms: the 'effusive' or 'wet' form, and the 'non-effusive' or 'dry' form.

The early symptoms for both forms of FIP - fever, lethargy, loss of appetite and weight loss - are symptoms which could easily point to any number of conditions. This is why it is so difficult to diagnose FIP at an early stage. As the disease progresses, however, the appearance of other symptoms can assist in the diagnosis of either effusive (wet) or non-effusive (dry) FIP.

FIP does not behave like other infectious diseases. It is what is called an immune-mediated disease, meaning that it somehow triggers the immune system into attacking the body rather than fighting the real bad guys. Antibodies are produced to bind the virus but the macrophages which are then supposed to engulf and destroy the antibody-bound virus fail to work properly; the antibody-bound virus is instead circulated throughout the body, accumulating in the walls of the blood vessels, causing severe inflammation. This inflammation, in turn, can cause thick, yellow, protein-rich fluid to seep through the blood vessel walls and accumulate in the abdomen (causing abdominal distension) and/or chest (causing breathing difficulties). With swelling of the abdomen, the cat will look fat but, to the touch, you will be able to feel the cat's vertebrae and hip bones. With fluid accumulation in the chest, breathing difficulty may be the only symptom. Sometimes, the fluid accumulates in the membrane surrounding the heart (pericardial sac), causing heart problems.

This accumulated fluid is termed an 'effusion' and gives effusive (wet) FIP its name.

Other common symptoms with wet FIP are problems with the eyes (clouding, redness or haemorrhaging), changes to the nervous system (causing abnormal behaviour or even seizures) and the liver (causing jaundice).

Alternatively, the FIP virus can trigger an inflammatory response which results in clusters of inflammatory cells (termed 'granulomas') accumulating at various sites around the body, commonly in abdominal organs such as the liver or kidneys, as well as the eyes and the nervous system. Symptoms that develop then depend on which organ or body system is affected. Since there are no effusions in this type of FIP, its name is 'non-effusive' (dry) FIP.

Symptoms with dry FIP are often very vague, making it very hard to diagnose. A persistent fever which doesn't respond to antibiotics, ongoing weight loss and lethargy could point to dry FIP. However, as the disease progresses and granulomas develop, various symptoms will develop - quite what these symptoms will be depend on which organ is being affected and how. As with wet FIP, the dry form also tends to affect the eyes and nervous system in a lot of cases.

Wet FIP is more common than dry FIP, accounting for something in the region of 60-70% of FIP cases.

The circulation of the antibody-bound virus complexes speeds up the progress of the disease and makes treatment very difficult. FIP usually progresses very rapidly, ultimately resulting in death.

The only bit of good news is that most cats will already have antibodies against the corona virus, including FIP.

How is FIP diagnosed

The only definitive way to diagnose FIP is via a tissue biopsy but in the majority of cases, the cat will be too ill to undergo this procedure. Hence, diagnosis of FIP is often only confirmed in a post mortem examination.

Without a tissue biopsy, diagnosis becomes more difficult.

Checking for antibodies to the corona virus would seem to be an obvious test. Unfortunately, though, such testing cannot distinguish between the ordinary corona virus (which causes few to no problems) and the mutated version which causes FIP. It also cannot be used to rule out FIP because approx. 10% of cats with FIP are found to have no detectable antibodies to the corona virus. The likelihood is that these cats are simply too sick to produce any antibodies.

Instead, if FIP is suspected, a thorough clinical examination is undertaken, including an eye examination and a neurological assessment. The more symptoms consistent with FIP found, the more probable that the cat has FIP.

If there is a fluid accumulation in the abdomen and / or chest, this can be analysed and will assist with diagnosis. Although the presence of fluid per se does not indicate FIP (since there are other conditions that present with this symptoms), the fluid that accumulates with FIP has quite individual characteristics: it is usually thick, yellowish in colour and, on further analysis, will show a very high protein content and low cell count.

Blood tests can assist in two ways - first of all by helping to rule out other possible causes for the symptoms, and secondly by looking for changes in the composition of the blood that are consistent with FIP. For instance, in many cases the number of lymphocytes (a type of white blood cell) is low, there is mild anaemia and the blood protein levels are very high. In some cases, bilirubin (pigment from old red blood cells) levels are high. Such changes do not definitively diagnose, but do increase the likelihood of, FIP.

Note that many of these abnormalities aren't always present in the early stages of FIP. Hence, if tests come back with normal results it can be beneficial to repeat them at a later date.

So how do you treat FIP?

Unfortunately, there is no cure for FIP. Typically, once clinical signs develop, the cat may live for only a few days, or a few weeks at most. In a few cases, adult cats with wet FIP have lived for six to eight months and some with dry FIP have survived a year or more.

Treatment can be given to relieve symptoms, for example anti-inflammatories and appetite stimulants. However, bearing in mind that FIP is a fatal disease, euthanasia may eventually be the kindest, most humane thing to do to relieve suffering.

A report published in 2004 described the use of recombinant feline interferon and glucocorticoids for the treatment of FIP. Although the study only involved 12 cats, the results were encouraging because they did suggest that such treatment can be effective in select cases of FIP. Feline interferon is available in the UK but is fairly expensive. This course of action can be discussed with your vet.

Is it possible to vaccinate against FIP?

There is a vaccine available in the United States (not in the UK) but its efficacy has not yet been proven, with different studies producing very different results. Hence, it cannot be relied upon to prevent the disease. In addition to this, it has only been licensed for use in kittens over 16 weeks of age - by which time most kittens will already be infected.